If you can "salvage" the hole and tap it you should be able to get a threaded insert into it. If necessary you can screw a set screw into it deep enough so that you can still use a standard bolt. You may have to use thread sealer on the set screw or the bolt to prevent any leaks. All is not lost, at least not yet. Worst case, you oversize the hole, tap it for an oversize bolt, cut it off flush, and then drill and tap the bolt to get yourself back to standard. No welding. To make sure you are centered you may have to attach the transmission and drill or centerpunch through it with a transfer punch to mark your hole.